Medical devices include devices designed to be implanted into a patient. Some examples of these implantable medical devices (IMDs) include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization therapy devices (CRTs), and devices that include a combination of such capabilities. The devices can be used to treat patients or subjects using electrical or other therapy or to aid a physician or caregiver in patient diagnosis by monitoring a patient condition. The devices can include one or more electrodes in communication with one or more sense amplifiers to monitor electrical cardiac activity within a patient, and can include one or more sensors to monitor one or more other internal patient parameters. Other examples of IMDs include implantable diagnostic devices, implantable drug delivery devices, or implantable neural stimulation devices.
Some IMDs detect events by monitoring electrical cardiac activity. In CFM devices, these events can include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs can detect abnormally slow heart rate, or bradycardia. In response to an abnormally slow heart rate, some CFM devices deliver electrical pacing stimulation energy to induce cardiac depolarization and contraction. The stimulation energy is delivered to provide a depolarization rate that improves hemodynamic function of the patient.
Normally, a patient's heart rate changes in response to a change in metabolic or physiologic need (e.g., during exercise). However, some patients' atrial depolarization rate may not adequately change in response to such changes. This condition is sometimes called chronotropic incompetence (CI). Examples of cardiac devices that can predict or recognize a chronotropically incompetent condition can be found in Scheiner et al., U.S. Pat. No. 7,142,920, entitled CHRONOTROPIC STATUS MONITOR FOR IMPLANTABLE MEDICAL DEVICE, which is hereby incorporated herein by reference in its entirety.
Pacemaker devices, such as dual-chamber devices, can use a rate-responsive pacing mode to treat patients with CI. In a rate-responsive, or rate-adaptive, pacing mode, information from a patient physical activity level sensor can be used by a pacemaker to adjust a patient heart rate. Some devices use more than one sensor to improve assessment of patient physical activity level, such as a quickly-reacting unspecific sensor (e.g., an accelerometer) and a more specific metabolic sensor (e.g., a respiration sensor).
In addition to CI, decreased heart rate variability can be associated with adverse cardiac outcomes including myocardial infarction or rapid progression of atherosclerosis and death from heart failure. Bakken et al., in U.S. Pat. No. 7,133,718, entitled METHOD AND APPARATUS FOR TEMPORARILY VARYING A PARAMETER IN AN IMPLANTABLE MEDICAL DEVICE, mention using a heart rate histogram to determine a heart rate profile. Bakken et al. also mention using the determined heart rate profile with a target rate profile to determine whether to use a rate variation feature of a pacemaker, such as an exercise simulator.